The overall goal of this research project is to investigate stages and mechanisms in the development of effective passive avoidance learning. Specifically, an effort has been made to demonstrate the presence of a passive avoidance deficit in young animals and to determine whether this deficit is due to deficiencies in response inhibition or deficiencies in learning to associate cues with punishment: 1. Passive avoidance development in the cat: kittens (25 and 50 days old) have been tested on a pre-trial passive avoidance task. 2. Active avoidance development in the cat: kittens (25 and 50 days old) have been tested on a step-up active avoidance task. 3. Passive avoidance development in the rat: rats of 18, 21, 25, and 100 days old were examined for entry latencies to the black side of a white-black box where they had been previously shocked. Presence of "familiarization" trials was used as a variable. 4. Passive avoidance development in the rat: rats 17, 20, 22, 25, 34 and 88 days of age are being tested on an operant task disrupted by fear conditioning. 5. Development rates will be manipulated through thyroidectomy or administration of pituitary growth hormone. Passive avoidance will be assessed at a number of chronological ages.